1. Field of the Invention
The present invention relates to the structure of a light-emitting diode and the method of manufacturing the same, and more particularly to the structure of a white LED and its manufacturing method, which includes at least two energy bandgaps.
2. Description of Prior Art
Conventional LEDs normally emit pure monochromatic light due to the characteristics of single peak wavelength and full width of half maximum (FWHM). For example, an AlGaAs-based LED emits red light and a GaP-based LED emits green light. Moreover, by using different materials or different component ratios in the same system, LEDs of different colors and different brightness can be obtained. For instance, adjusting the P to As ratio in AsGaP or the Al to Ga and In ratio in AlGaInP can generate a highly bright AsGaP-based or AlGaInP-based LED which can emit red, yellow or green light. However, the above-described LED can only emit monochromatic light.
In practical terms, two kinds of LEDs that emit different colors, such as a red LED (R) and a green LED (G), can be combined to form a multi-colored LED display panel through adjusting the intensity ratio of red light and green light. Alternatively, three kinds of LEDs that emit different colors, such as a red LED (R), a green LED (G) and a blue LED (B), can be combined to form a full-colored display array of LEDs through adjusting the intensity ratio of red light, green light and blue light. Since the full-colored display panel can emit white light, the use of LEDs can be signal-indicating and advertising, as well as illumination. FIG. 1 illustrates a plan diagram for white LED light output, the pattern of white is No. NLK2015, manufactured by Nichia Chemical Industry of Japan. It is found that each pixel 10 consists of two red LEDs (R), two green LEDs (G) and one blue LED (B). Therefore, such a white light-emitting device is costly. Further, the design of the drive system is very complex because at least four electrical terminals are needed to change currents respectively flowing through different colors of LEDs, so that the intensity of each LED can be adjusted to produce white light. If one of the five LEDs suffers from light-output degradation, the color balance of the white light-emitting device is broken.
In order to reduce the cost, Nichia Chemical Industry provides another white LED lamp that does not require combining LEDs that emit red light, green light and blue light, respectively. Referring to FIG. 2, the white LED lamp includes a GaInN/GaN blue LED chip 12 and fluorescent material. The steps for fabricating the white LED lamp include placing an LED chip 12 on a metal pin 15a, connecting the surface electrode of the LED chip 12 to another metal pin 15b, filling fluorescent material 14 into the trough of the metal pin 15a to cover the LED chip 12, and then packaging the LED chip 12 and the ends of the metal pins 15a and 15b with packing material 16. The white LED lamp produces white light by compounding blue light emitted by the LED chip 12 and yellow light, which is generated by exciting the fluorescent material 14 with the blue light. This approach significantly reduces the cost of the white LED lamp and simplifies the drive system. However, the lifetime of the fluorescent material is only about 5000.about.10000 hours, far less than that of a white LED chip that is about 100000 hours, thus limiting the lifetime of such a white LED lamp.